The basic methods to control ignition system timing are as follows:

CENTRIFUGAL ADVANCE (controlled by engine speed)

VACUUM ADVANCE (controlled by intake manifold vacuum and engine load)

COMPUTERIZED ADVANCE (controlled by various sensors - speed, temperature, intake, vacuum, throttle position, etc.)

Centrifugal Advance

Centrifugal advance makes the ignition coil and spark plugs fire sooner as engine speed increases, using spring-loaded weights, centrifugal force, and lever action to rotate the distributor cam or trigger wheel. Spark timing is advanced by rotating the distributor cam or trigger wheel against distributor shaft rotation. This action helps correct ignition timing for maximum engine power. Basically the centrifugal advance consists of two advance weights, two springs, and a advance lever.

During periods of low engine speed, the springs hold the advance weights inward towards the distributor cam or trigger wheel. At this time there is not enough centrifugal force to push the weights outward. Timing stays at its normal initial setting.

As speed increases, centrifugal force on the weights moves them outwards against spring tension. This movement causes the distributor cam or trigger wheel to move ahead. With this design, the higher the engine speed, the faster the distributor shaft turns, the farther out the advance weights move, and the farther ahead the cam or trigger wheel is moved forward or advanced. At a preset engine speed, the lever strikes a stop and centrifugal advance reaches maximum.

The action of the centrifugal advance causes the contact points to open sooner, or the trigger wheel and pickup coil turn off the ECU sooner. This causes the ignition coil to fire with the engine pistons not as far up in the cylinders.

Vacuum Advance

The vacuum advance provides additional spark advance when engine load is low at part throttle position. It is a method of matching ignition timing with engine load. The vacuum advance increases FUEL ECONOMY because it helps maintain idle fuel spark advance at all times. A vacuum advance consists of a vacuum diaphragm, link, movable distributor plate, and a vacuum supply hose.

At idle, the vacuum port from the carburetor or throttle body to the distributor advance is covered, thereby NO vacuum is applied to the vacuum diaphragm, and spark timing is NOT advanced. At part throttle, the throttle valve uncovers the vacuum port and the port is exposed to engine vacuum. The vacuum pulls the diaphragm outward against spring force. The diaphragm is linked to a movable distributor plate, which is rotated against distributor shaft rotation and spark timing is advanced.

The vacuum advance does not produce any advance at full throttle. When the throttle valve is wide open, vacuum is almost zero. Thus vacuum is NOT applied to the distributor diaphragm and the vacuum advance does NOT operate.

Computerized Advance

The computerized advance, also known as an electronic spark advance system, uses various engine sensors and a computer to control ignition timing. The engine sensors check various operating conditions and sends electrical data to the computer. The computer can change ignition timing for maximum engine efficiency.

Ignition system engine sensors include the following:

ENGINE SPEED SENSOR (reports engine speed to the computer)

CRANKSHAFT POSITION SENSOR (reports piston position)

THROTTLE POSITION SWITCH (notes the position of the throttle)

INLET AIR TEMPERATURE SENSOR (checks the temperature of the air entering the engine)

ENGINE COOLANT TEMPERATURE SENSOR (measures the operating temperature of the engine)

DETONATION SENSOR (allows the computer to retard timing when the engine knocks or pings)

INTAKE VACUUM SENSOR (measures engine vacuum, an indicator of load)

The computer receives different current or voltage levels (input signals) from these sensors. It is